Kinkly explains Fetters

Fetters are a little bit like handcuffs, but with wider metal cuffs and typically a longer chain between them. They also go around ankles rather than wrists.

Fetters are what you usually see tied to the "ball and chain." They can be tied to ankles and then chained to objects in the floor to limit the wearer's area of movement. Wearing fetters is enough to limit someone's freedom of movement: the heavy chains and shorter length reduces your ability to take normal steps.

Using fetters as restraints is more showy than practical. First, they are expensive; second, you can get the exact same effect with cuffs and a rope or smaller chain. For a strong effect on the mind of the wearer, the weight of the iron around the ankles as well as the heavy chain can work wonders.